Adam Back is a British cryptographer and businessman best known for inventing Hashcash, a proof-of-work system proposed in 1997 to fight email spam, which Satoshi Nakamoto cited in the 2008 Bitcoin whitepaper. He is a co-founder and the chief executive of Blockstream, a company that builds infrastructure for Bitcoin, including sidechain, satellite and hardware wallet products.
Back was an active member of the cypherpunk movement of the 1990s, a loose group of cryptographers and activists who promoted privacy through cryptography. He holds a PhD in computer science from the University of Exeter. In the Bitcoin community he is regarded as an influential voice on protocol development and scaling, and he has often taken part in public debates about how Bitcoin should evolve.
Adam Back in plain terms
Long before Bitcoin, inboxes were flooded with junk email because sending a message cost nothing. Back's idea was to make each email carry a small "stamp" that required a computer to do a bit of puzzle-solving work. For a normal person sending a few emails, the cost would be invisible. For a spammer sending millions, it would add up fast.
That idea of proving you spent effort turned out to be useful for money too. Bitcoin uses the same kind of puzzle to decide who adds the next page to its shared record book, which is why Back's work is often mentioned in the story of how Bitcoin was built.
Hashcash
Back announced Hashcash on the cypherpunks mailing list in 1997 and later described it in a 2002 paper on denial-of-service countermeasures. A sender's computer must find a value that, when combined with the message details and run through a cryptographic hash function, produces an output starting with a set number of zero bits. Finding such a value takes many attempts, but checking it is instant. This asymmetry is the heart of proof of work. Bitcoin adapted the approach, using repeated SHA-256 hashing and adding a mining difficulty that adjusts to keep blocks arriving at a steady pace, combined with rewards for the miners who do the work. Earlier, in 1992, Cynthia Dwork and Moni Naor had proposed a related pricing-function idea, and Back has acknowledged this prior work.
Career and Blockstream
In the late 1990s and early 2000s Back worked in privacy technology, including at the Canadian company Zero-Knowledge Systems, which developed an anonymity network. According to emails he has published, Satoshi Nakamoto contacted him in August 2008 to check how to cite Hashcash, and Back pointed Satoshi towards Wei Dai's earlier b-money proposal. Back became more directly involved with Bitcoin in the early 2010s and co-founded Blockstream in 2014 with other developers, including Gregory Maxwell and Pieter Wuille. Blockstream's projects have included:
- Liquid Network. A federated sidechain launched in 2018 for faster settlement and issued assets, used mainly by exchanges and traders.
- Blockstream Satellite. A service broadcasting the Bitcoin blockchain from satellites so users can run nodes without regular internet access.
- Lightning development. Work on software for the Lightning Network, a payment layer built on top of Bitcoin.
- Hardware and mining. A hardware wallet and mining and hosting services.
Views and public role
Back has been a prominent participant in Bitcoin's technical debates. During the block size dispute of 2015 to 2017 he argued for keeping the base layer conservative and scaling through upgrades such as Segregated Witness and second-layer systems, a position that was opposed by advocates of larger blocks. Earlier, the 2014 sidechains paper he co-authored proposed ways to move bitcoin between chains. He is an outspoken supporter of Bitcoin as a long-term store of value and often comments on cryptography, privacy and regulation on social media and at conferences. He has also worked on privacy techniques for Bitcoin, including early ideas that fed into Confidential Transactions, a method of hiding payment amounts while still letting the network check that no coins were created from nothing; the technique is used on the Liquid Network.
Speculation about Satoshi Nakamoto
Because of his early work on proof of work and his cypherpunk background, Back has been named in online speculation about the identity of Satoshi Nakamoto. He has repeatedly and publicly denied being Satoshi. No verified evidence links him to Satoshi's identity, and the question of who created Bitcoin remains unresolved.